In typical refinery operations, vacuum gas oil produced in a vacuum distillation column is fed to a catalytic cracking zone to produce naphtha and distillate fuels which are higher in value. When operating toward this goal, the catalytic cracking zone is operated under a given set of conditions to increase the yield of these desired products. During the cracking reactions, carbon, in the form of coke, deposits on the surface of the catalyst. The coke is burned from the catalyst in a regeneration zone in the presence of oxygen to promote combustion. Because the amount of coke is so great and combustion so intense, catalyst coolers are sometimes used to reduce the catalyst temperature prior to being reintroduced into the catalytic cracking zone.
However, when the goal is to produce light olefins, such as ethylene and propylene, catalysts and process conditions conducive to the catalytic cracking processes used to reduce the average molecular weight of gas oil or residual oils (“resids”) are not optimum for converting the naphtha from a catalytic cracking zone to light olefins, such as ethylene and propylene. There is a benefit to separating the catalytic cracking zone from an olefin producing zone.
When the reaction zones and the regeneration zones for the catalytic cracking and olefin production zones were separated, it was found that less coke is deposited on the surface of the olefin producing catalyst because of the nature of the feedstock to the process. Light hydrocarbons, such as naphtha or light naphtha, produce significantly less coke than cracking of resids or gas oils. Combustion of the coke may not be sufficient to supply enough heat to an olefin producing zone for efficient olefin production. It was found that there was a need in the art for a way to separate the catalytic cracking and olefin producing functions, while maintaining a suitable yield of light olefins. There was a further need in the art for a process or reactor system that generates light olefins in a more energy efficient manner.